Plasma-etchable insulating material can be structured in a plasma etching process known as the DYCOstrate.RTM. process. Such structures can be openings which pass through the insulating material as through holes, or can be blind holes which merely extend into the insulating material. Through holes and blind holes can also be in the form of elongated holes resembling grooves which extend in different and varying depths in the insulating material with straight or curved edges. During plasma etching, the structures are simultaneously produced with high precision in the insulating material. This process is economical because it permits inexpensive, rapid formation of very varied structures and patterns in the insulating material. The plasma-etchable insulating material is constituted by organic, dielectric films such as, for example, polyimides or cyanateester resin films, as well as films of liquid crystal polymers (LCP), etc.
In order to be able to etch such insulating material with a locally controlled plasma, the material is coated with a plasma etching-resistant material. Such plasma etching-resistant materials can be metal layers of copper of aluminum, i.e., layers made of plasma etching-resistant electrically conductive material. The applied, plasma etching-resistant material is provided with openings extending therethrough to the insulating material, exposing the insulating material so that the plasma can interact through the openings with the insulating material and etch it.
Since, for thermal reasons and for reasons of the plasma etching rate associated therewith, directional plasma etching (reactive ion etching) does not appear appropriate for the production of film circuit boards, use is made of isotropic plasma etching, i.e., insulating material is uniformly removed everywhere the plasma has access to it. Thus, the insulating material can also be removed under the edges of the openings in the plasma etching-resistant material, i.e., below the plasma etching-resistant material. This under-etching or under-cutting means that the edges of the openings project in quasi-insulated manner in the space from the solid or rigid plasma etching-resistant material following plasma etching and that the insulating material has etched-back cavities.
The use of plasma etching-resistant conductive material firmly attached to the plasma-etchable insulating material has proved advantageous in the manufacture of printed circuit boards and film circuit boards. Such conductor material can be applied to one or both sides of the insulating material in the form of clad copper layers. Following the plasma etching of openings in the insulating material, the conductor material can be structured or patterned to form electrical current paths in subsequent processing stages and the openings in the insulating material can be plated with metal to form interfacial connections between different planes of structured material.
However, etch-backs, i.e., the projecting edges or webs of plasma etching-resistant material around openings in the insulating material prove disadvantageous for the additional subsequent processing stages. Thus, the following problems occur in the electro-deposition of copper:
The regions under the projecting webs around openings in the insulating material is electrically shielded during the electro-deposition of metal layers, so that only small quantities of copper are deposited. The result is, for example, that the reliability of interfacial connections is not ensured. PA1 The etched-back cavities of the openings in the insulating material cannot be adequately cleaned by techniques such as degassing or washing out. In electro-deposition with a plurality of successive baths, this leads to inadequate results and to carrying-over of chemicals from one bath to the next. PA1 The webs around the openings in the insulating material are thin and easy to deform mechanically. For example, they are bent up and deformed by ultrasonic baths when such baths are used as the cleaning medium. This leads to undesirable results during subsequent photochemical process stages.
One possibility for removing the projections at etch-backs is to press the webs around the plasma-etched openings by pressure action into the openings in the insulating material such as described, for example in U.S. Pat. No. 4,472,238. This patent uses two-sided, copper-clad polyimide films such as Pyralux.RTM. DuPont F9111, or copper foil-coated Kevlar.RTM. as the plasma etching-resistant material and plasma-etchable insulating material. Copper edges projecting over the 76-254 .mu.m diameter holes in the nonconductive polyimide or Kevlar films are pressed into the holes at 124 atm.
This process suffers from serious disadvantages. The finer the plasma-etched structures which are under-etched, the greater the overpressures which must be applied in order to press the webs into the openings. This leads to excessive mechanical stressing and undesirable dimensional changes and is consequently technically impractical.
Another possibility for removing the under-etchings is to press the webs around plasma-etched openings into the openings by material bombardment. In printed circuit board technology, such a process is referred to as a jet scrubber process in which, typically, an aqueous solution of pumice powder is sprayed under high pressure against the projecting edges of openings so that the webs are pressed into the openings.
However, this process also suffers from serious disadvantages. There is a mechanical cold deformation of the pumice powder-bombarded surfaces which leads to undesired mechanical stresses and dimensional changes. The process is only usable with very thin layers of plasma etching-resistant conductor material. Also, there is a partial incorporation of the pumice powder and knocked-off particles of plasma etching-resistant material into other areas of the printed circuit board or film circuit board to be produced which, in turn, leads to disturbing effects such as impurities, electrical short-circuit contacts and the like. Thus, this process is technically impractical.